For those Democrats who still revere the memory of Franklin Roosevelt, Tuesday night was a time for many lusty choruses of his theme song, “Happy Days Are Here Again.”

In 48 hours, the Democrats have gone from the fetal crouch to giddy exuberance. New Jersey offered few surprises as former Goldman Sachs executive Phil Murphy bridged his Wall Street background to cruise to any easy victory over Chris Christie’s lieutenant governor, Kim Guadagno.

But in Virginia, Democrats had lived in mortal terror that the seemingly bland campaign of Lt. Gov. Ralph Northam would be no match for the Donald Trump-esque posturing of former Washington lobbyist and Republican national chairman Ed Gillespie.

Instead, Northam carried the state by a larger margin than Gov. Terry McAuliffe in 2013 or Hillary Clinton in 2016. Instead of fulminating about Latin American gangs, Gillespie probably would have done better had he emulated Haley Barbour, another former lobbyist and party chairman, who was elected Mississippi governor in 2003 promising to be a tireless lobbyist for his state.

Many will be tempted to overhype Tuesday night’s results (including a Maine vote to expand Medicaid under Obamacare) as a precursor to a Democratic takeover of the House in 2018.

What is clear is that the northern Virginia suburbs have become as safely and as permanently Democratic as any major urban area. Northam’s vote percentage in Fairfax County (66.6 percent) was actually greater than New York Mayor Bill de Blasio’s total (66.5 percent) in winning a second term. The anti-Trump fervor in northern Virginia should be particularly worrisome for endangered two-term GOP Rep. Barbara Comstock, who represents parts of Fairfax County and other Washington-area bedroom communities.

But there are also reasons for caution in extrapolating too much from the limited 2017 returns. Odd-year elections are like a crooked roulette wheel in a small town — everyone still bets because it’s the only game in town.

Here are just a few of the still-unknown factors that could shape the 2018 elections:

Will the economy still be healthy or have we dipped into the recession that is inevitable someday? Will the Republicans pass a tax cut? If so, will middle-class voters feel that they have more take-home pay when the new 2018 withholding schedules are calculated?

What will be Donald Trump’s approval rating? Has the president risen above the 40-percent mark or drooped into pre-resignation Richard Nixon territory? Will Americans be engulfed in a new war on the Korean Peninsula or in the Middle East where the Saudis (egged on by Jared Kushner) seem to want to foment a crisis?

Has Trump opened up a new front in the never-ending culture wars — again forcing Americans to choose sides as they had to do with kneeling players in the NFL? Will there be another terrorist attack like New York? If so, will Trump fan the flames of controversy or offer a more traditional presidential reaction?

Will the Democratic civil war between the establishment and Bernie Sanders militants lead to divisive congressional primaries and unelectable nominees? Will a continuing wave of Republican retirements put more once-safe GOP House seats into play?

And, perhaps most important of all, what will be the status of Robert Mueller’s investigation? Has he wrapped matters up with a few high-profile cases like Paul Manafort and maybe Michael Flynn? Or in November 2018, will Mueller be getting ever closer to Trump and his family?

That’s a lot of wills that can lead in a lot of ways. Anyone who thinks that the next 12 months will be a straight-line projection from today simply hasn’t been paying attention.

But Tuesday night’s Democratic wave has important short-run implications. There will be many rational Republicans wondering if Sen. Jeff Flake doesn’t have the right idea about how to behave in an age of Trump. Suddenly, the strategy politics of writing off a large chunk of college-educated white voters, millennials and minorities seems like a dicey electoral proposition.

Just 48 hours ago, the biggest political story of the week was the publication of former interim Democratic chair Donna Brazile’s book, “Hacks: The Inside Story of the Break-ins and Breakdowns That Put Donald Trump in the White House.”

By revealing new details of a deliberate tilt toward Hillary Clinton by the DNC under Debbie Wasserman Schultz, Brazile’s campaign memoir aroused another wave of pro-Sanders conspiracy theories claiming that the 2016 primaries had been rigged. But, in truth, Brazile’s story underscores that the DNC was too broke and too inept to successfully rig anything, no matter how heavy-handed Wasserman Schultz’s efforts were.

To a large extent, the Democratic National Committee in 2016 was like a failed state on a map of the world. It looks impressive when all that you see on the map are its name and its borders. But its ability to govern itself and assert power over events can easily be exaggerated.

Election Night 2017 proved to be an apt time to review the Faustian bargain that another party chairman, the GOP’s Reince Priebus, made with Trump in 2016. By embracing the bilious billionaire and cutting the Republican primary race short, Priebus was convinced that he had cleverly co-opted and tamed the outsider who took over the party.

Now a year after Trump’s unlikely victory, nobody has tamed the tiger. Priebus, in fact, has been carted off by Trump to the dustbin of history. And the Republicans are paying the price for their continuing alliance with an unpopular president whose only accomplishments have been spreading division and inspiring disdain.